Max Zs

[Rocboni]

Hello,

Having a bit of difficulty completing the below assignment, wondered if anyone can expand on this?

Explain in detail the relationship between the Protective Device and the Earth Fault Loop Impedance, Maximum Zs for the protective device and how this is calculated, Maximum disconnection Times and the amount of current required to operate the device within the maximum disconnection time. Min 300 words

Not looking for someone to write it for me, just some points which I can expand on.

Thanks

 

[Pete999]

You see a prime example of a guy asking a perfectly viable question, look at his profile and it's all there, what he has done what quals he has, well done mate, I'll let someone with more savvy than me answer your question, I could answer it but not right now

 

[Rocboni]

Thanks Pete,

Came up with the following, interested to know your thoughts...

In order to meet the requirements of table 41.1 circuits must have a sufficient earth path in order for sufficient fault current to flow in the event of a fault, this ensures that the protective device disconnects in the specified times. This fault can be either a short circuit where excessive current flows due to an extremely low resistance between phase and phase, (it should be noted that a short circuit on a 3 phase system will be significantly bigger than a single phase fault as the voltage is higher i.e. 415V compared to 230V) or fault current where current flows through an untended path such ans the circuit protective conductor.



Earth fault loop impedance is a measurement of the resistance of the path which fault current flows when a fault of low impedance occurs between the phase conductor R1 and earth R2, if the impedance is high the lower the resulting fault current will be therefore the longer the protective device takes to operate.



In order for the protective device to operate fast enough the loop impedance must not exceed that specified for the device (tab 41.3-41.4) these values have been calculated so that sufficient current flows to earth, tripping the protective device in the below disconnection times.

A Ze, external earth fault loop impedance must be obtained first, this can be obtained either by enquiry if the supply has not been installed or by measurement. This gives the loop impedance of the supply(earth path back to transformer)



If an earth fault loop impedance is taken at for example a socket, this already includes the Ze, Zs is defines as Ze+(R1+R2)



The Maximum earth fault loop impedance value for a particular device can be obtained from the following formula



Zs= Uo/Ia



where;

Zs = earth fault loop impedance

Uo = system voltage

Ia = disconnection current in Amps obtained from the time fault current characteristics charts in appendix 3



In order to comply with 411.3.2 Bs7671 the maximum disconnection times are as follows,



Circuits rated 32A or less on a TN system must disconnect in no more than 0.4s

Circuits rated over 32A on a TN system must disconnect in no more than 5s

Circuits rated 32A or less on a TT system must disconnect in no more than 0.2s

Circuits rated over 32A on a TT system must disconnect in no more than 1s



Breaking capacity of a protective device is the maximum current of which the circuit breaker can disconnect without becoming permanently damaged or destroyed. This can be verified by PFC this can be obtained wither by measurement or calculation by dividing the the voltage by the Ze e.g. 415/0.05 = 8300A therefore a MCB of only 6kA would not be suitable and a MCB of 10kA should be used. A device shall be capable of breaking, and for a CB making, the fault current upto and including the PFC. Regulation 432.3



Two examples of Time/fault current characteristics are attached. (hand drawn)

 

[Leesparkykent]

The over-riding requirement is that sufficient fault current must flow in the event of an earth fault to ensure that the protective device cuts off the supply before dangerous shock can occur.
Maximum earh fault loop impedance values are calculated using the instantaneous tripping current, the line voltage to earth and Cmin.
Minature circuit breakers typicaly come as B,C and D types although others are avaliable but less common like K and Z
Type B's have an instantaneous tripping current of between 3 and 5 In
Type C's have an instantaneous tripping current of between 5 and 10 In
Type D's have an instantaneous tripping current of between 10 and 20 In
Cmin The minimum voltage factor takes account of the fact that the voltage of the electricity supply to an electrical installation varies depending on time and place,
changing of transformer taps and other considerations.
Amendment No. 3 to BS 7671:2008 gives Cmin the value of 0.95 where the low voltage supply given in accordance with the ESQCR.
So for example the maximum tabulated earth fault loop impedance given is 1.37 Ohms for a 32A type B circuit breaker at 100% In
32Ax5In=160A
230V/160A=1.43 Ohms
1.43 Ohmsx0.95(Cmin)=1.37Ohms
The instantaneous tripping current is the minimum value of current that causes the circuit breaker to trip without delay (i.e., in less than 100 ms) and also meeting disconnection times.

Edit; took me so long to type just noticed you've posted What you've put looks good but don't forget Cmin on the calculations.

 

[Rocboni]

We use an online program which compares the submitted document to online and other peoples work giving the lecturer a percentage familiarity, I hope it doesn't pick up on this!

 

[Leesparkykent]

We use an online program which competes the submitted document to online and other peoples work giving the lecturer a percentage familiarity, I hope it doesn't pick up on this!

I wouldn't of thought so mate as its in a closed section with no access to bots like google and bing etc

 

[happysteve]

Yeah, don't worry about turnitin, won't get in here.

You've covered most of the things in your thoughts above. I guess the one thing I'd like to get across if it was me doing the exercise is that ultimately, it's disconnection times stated in table 41.1, and the two regs at the top of the next page, that are the start of the story. I genuinely don't know where those times come from (I suspect a smoke packet was involved somewhere). But protective devices must meet those times. Generally, the higher the current, the faster the disconnection. Just how fast is defined in British Standards (e.g. BS 88, BS EN 60898 etc). So the standards define the minimum current required for a certain time. Now we know the minimum current, we just need Ohm's Law - and a bit of jiggery-pokery with the Cmin fudge factor - to determine the maximum Zs value. So in practice we use max Zs tables to ensure compliance, but ultimately it's disconnection times we're required to meet.

Another thing to think about is the effect of temperature. The max Zs values in the Regs are different from those quoted in the OSG and GN3 - you could explain why, I'm sure.

(Very minor thing: you mention 415V - this is from pre-1995, it's U=400V now.)

Good stuff.